Field of the Invention
The present invention relates to nontoxic gas generating compositions that when combusted rapidly generate gases that are useful for actuating vehicle occupant restraint systems in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates and sustained combustion, but also a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.
The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature. See for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588, 5,035,757, 5,386,775, and 5,872,329, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants often contain ingredients such as an oxidizer to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing sufficient quantities of a nontoxic gas to inflate the safety device at an acceptable rate.
The use of phase stabilized ammonium nitrate is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable.
Often, gas generant compositions incorporating phase stabilized or pure ammonium nitrate exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NOx, for example, depending on the composition of the associated additives such as plasticizers and binders. In addition, ammonium nitrate contributes to poor ignitability, lower burn rates, and performance variability. Several known gas generant compositions incorporating ammonium nitrate utilize well-known ignition aids such as BKNO3 to solve this problem. However, the addition of an ignition aid such as BKNO3 is undesirable because it is a highly sensitive and energetic compound, and furthermore, contributes to thermal instability and an increase in the amount of solids produced.
Certain gas generant compositions comprised of ammonium nitrate are thermally stable, but have burn rates less than desirable for use in gas inflators. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.4 inch/second (ips) or more at 1000 psi. Gas generants with burn rates of less than 0.40 ips at 1000 psi do not ignite reliably and often result in xe2x80x9cno-firesxe2x80x9d in the inflator.
Yet another problem that must be addressed is that the U.S. Department of Transportation (DOT) regulations require xe2x80x9ccap testingxe2x80x9d for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, most propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.
The compositions described in U.S. Pat. No. 5,035,757 to Poole exemplify state of the art gas generant compositions that function well but produce relatively large amounts of solid combustion products. As a result, the gas produced is less than that produced by current state of the art xe2x80x9csmokelessxe2x80x9d gas generants. Thus, more gas generant and greater filtering requirements are required to facilitate operation of an airbag inflator.
On the other hand, compositions described in U.S. Pat. No. 5,872,329 to Burns et al. exemplify current state of the art xe2x80x9csmokelessxe2x80x9d gas generants. The combustion products are primarily gas with minimal formation of solids. The benefits include a reduction in the amount of gas generant required and reduced filtering requirements. However, certain compositions described by Burns may be disadvantaged by lower burn rates and a failure to sustain gas generant combustion. To overcome these disadvantages, a stronger and more robust inflator is often required to increase the operating pressure of the inflator and thereby improve the burn of the gas generant.
Accordingly, it would be an improvement in the art to provide the gas generant burn characteristics of compounds as described in U.S. Pat. No. 5,035,757 along with the capacity to produce more gas and less solids as typified by state of the art xe2x80x9csmokelessxe2x80x9d gas generants.
The present invention generally relates to gas generant compositions useful in actuating a vehicle occupant restraint system in the event of a motor vehicle accident. Application within a vehicle occupant restraint system includes actuation of a seatbelt pretensioner and/or inflation of an airbag. Other applications requiring gas generation are also contemplated, including fire suppression systems aboard aircraft and inflators for flotational devices, for example.
The above-referenced problems are reconciled by compositions containing 5-aminotetrazole nitrate (5ATN) as a fuel at about 25-100% by weight of the total composition. An oxidizer is selected from a group of compounds including phase stabilized ammonium nitrate, ammonium nitrate, potassium nitrate, strontium nitrate, copper dioxide, and basic copper nitrate. Other oxidizers well known in the art are also contemplated. These generally include but are not limited to inorganic oxidizers such as alkali and alkaline earth metal nitrates, nitrites, chlorates, chlorites, perchlorates, and oxides.
Standard binders, slag formers, and coolants may also be incorporated if desired.
A composition in accordance with the present invention contains by weight 25-95% 5ATN and 5-75% of an oxidizer. A more preferred composition consists of 55-85% 5ATN and 20-45% PSAN.
A method of formulating compositions of the present invention includes providing an excess amount of nitric acid (preferably 15.9M or less and preferably chilled at 0-20xc2x0 C.), and then, in the appropriate amounts, adding a nitratable fuel such as 5-aminotetrazole and at least one oxidizer to the nitric acid. The slurry is stirred until a damp or wet paste forms. The paste is then formed into the desired shapes and dried.